This invention relates to an apparatus for detecting a physical quantity acting as an external force, e.g., a force exerted on a working body, an acceleration exerted on a weight body, or a magnetism exerted on a magnetic body. Particularly, this invention relates to a signal processing circuit, a testing method, and a manufacturing method for a force sensor serving as a main part of such a detector, and a structure of this force sensor.
In recent years, there are proposed force sensors including, arranged on a semiconductor substrate, resistance elements having a property of the piezo resistive effect that the electric resistance varies in dependency upon a mechanical deformation to detect a force from changes in resistance values of the resistance elements. Further, acceleration sensors or magnetic sensors to which such force sensors are applied are also proposed. In either detector, a strain generative body partially having flexibility is used to detect a mechanical deformation produced in the strain generative body as changes in electric resistance of the resistance elements. A working body is provided for exerting a force on the strain generative body. If a weight body responsive to acceleration is used as the working body, an acceleration sensor is provided. Further, if a magnetic body responsive to magnetism is used as the working body, a magnetic sensor is provided.
For example, in U.S. Pat. No. 4,905,523, U.S. patent application Ser. No. 295,210, No. 362,399, No. 470,102, and No. 559,381, force/acceleration/magnetic sensors according to the invention of the inventor of this application are disclosed. Further, in U.S. Pat. application Ser. No. 526,837, a novel manufacturing method for a sensor of this kind is disclosed.
The force sensors disclosed in these patent materials can detect a direction and a magnitude of an external force applied to a predetermined working point on the basis of changes in resistance values of resistance elements formed on a single crystal substrate. If a weight body is formed at the working point, it is also possible to detect, as a force, an acceleration exerted on the weight body. Accordingly, this permits application as an acceleration sensor. Further, if a magnetic body is formed at the working point, it is also possible to detect, as a force, magnetism exerted on the magnetic body. Accordingly, this permits application as a magnetic sensor.
However, the conventional force sensors (or acceleration sensors, or magnetic sensors based on the same principle) have the problem that there may occur interference in the output characteristic with respect to respective axial directions of two-dimensions or three-dimensions. For example, in the case of the three dimensional acceleration sensor, components in the X-axis, Y-axis and Z-axis directions of an acceleration exerted on a predetermined working point must be independently detected, respectively. In the case of conventional sensors, however, these components in respective axial directions interfere with each other. As a result, a detected value of the component in one axial direction was influenced to some extent by detected values of components in other axial directions. Such an interference is not preferable because it lowers reliability of measured values.
With the above in view, a first object of this invention is to provide a signal processing circuit capable of obtaining correct detected values free from interference of the components in other axial directions.
In order to mass produce such sensors to deliver them on the market, it is necessary to conduct a test or inspection at the final stage of the manufacturing process. The test for the force sensor can be carried out relatively with ease. Namely, this test may be accomplished by applying a force of a predetermined magnitude to the working point in a predetermined direction to check a detected output at this time. However, the test for the acceleration sensor or the magnetic sensor becomes more complicated. Since the sensor body is in a sealed state, it is necessary to check detected outputs while actually exerting, from the external, acceleration or magnetism thereon. Particularly, in the case of the acceleration sensor, it is the present state that a vibration generator is used to give vibration to the sensor body to carry out a test. This results in the problem that the testing device becomes large, and a test for a dynamic acceleration of vibration is only conducted.
With the above in view, a second object of this invention is to provide a testing method capable of more easily testing a sensor having a working body of force such as an accelerating sensor or a magnetic sensor, and to provide a sensor having a function capable of immediately carrying out this testing method.
Further, conventionally proposed sensors using resistance elements has a problem in the case of carrying out high sensitivity measurement. For example, in the case of the acceleration sensor, it is sufficient for the purpose of utilizing collision detection of a vehicle, etc. to have a function to detect acceleration of the order of 10 to 100 G on the full scale. However, in order to detect a swing by hand of a camera, to conduct a suspension control for a vehicle, and to conduct a control for an antilock brake system for a vehicle, it is necessary to detect an acceleration of the order of 1 to 10 G. For carrying out such a high sensitivity acceleration detection, it is necessary to increase the weight of a working body having a function to produce a force on the basis of an acceleration. However, in the case of the structure of conventional sensors, it was difficult to enlarge the working boby.
In the case of the high sensitivity sensor, where a large force more than a predetermined limit is applied thereto, the danger that the semiconductor substrate may be damaged is increased. For this reason, it is necessary to provide, around the working body, a member for allowing displacement of the working body to limitatively fall within a predetermined range. This gives another problem that the structure becomes complicated.
Further, in the case of detecting force, acceleration, and magnetism, etc. exerted in three-dimensional directions, there would occur a difference between a detection sensitivity in a direction parallel to the surface of the semiconductor substrate and that in a direction perpendicular thereto. The fact that a sensitivity difference occurs in dependency upon the direction of detection is not particularly preferable for high sensitivity sensors.
With the above in view, a third object of this invention is to provide a sensor using resistance elements suitable for higher sensitivity physical quantity measurement, and a method of manufacturing such a sensor.
I Feature relating to the first object
To achieve the first object to provide a signal processing circuit capable of obtaining a correct detected value free from interference of the components in other axial directions, this invention has the following features.
(1) The first feature resides in a signal processing circuit for a sensor in which when an external force is exerted on a predetermined working point in an XYZ three-dimensional coordinate system, a mechanical deformation is produced on a single crystal substrate by the external force, the sensor detecting a component in the X-axis direction Ax, a component in the Y-axis direction Ay, and a component in the Z-axis direction Az of the external force exerted on the working point on the basis of electric signals Vx, Vy and Vz produced due to the mechanical deformation,
wherein coefficients K11, K12, K13, K21, K22, K23, K31, K32 and K33 are determined so that the relational equations expressed below hold between Ax, Ay, Az, Vx, Vy and Vz:
Ax=K11Vx+K12Vy+K13Vz
Ay=K21Vx+K22Vy+K23Vz
Az=K31Vx+K32Vy+K33Vz
and the values of terms of the right sides of the relational equations are computed by using analog multipliers, and a computation between respective terms of the right sides of the relational equations is performed by using analog adders/subtracters, thus to provide detected values Ax, Ay and Az from these computed results.
(2) The second feature resides in a signal processing circuit for an acceleration detector in which when an external force is exerted on a predetermined working point, a mechanical deformation is produced on a single crystal substrate by the external force, the detector detecting a component in an X-axis direction Ax and a component in a Y-axis direction perpendicular thereto of an external force exerted on a working point on the basis of electric signals Vx and Vy produced due to the mechanical deformation,
wherein coefficients K11, K12, K21 and K22 are determined so that the relational equations expressed below hold between Ax, Ay, Vx and Vy:
Ax=K11Vx+K12Vy
Ay=K21Vx+K22Vy
and the values of terms of the right sides of the relational equations are computed by using analog multipliers, and a computation between respective terms of the right sides of the relational equations is performed by using analog adders/subtracters, thus to provide detected values Ax and Ay from these computed results.
(3) The third feature resides in a signal processing circuit for a force sensor in which a plurality of resistance elements exhibiting the piezo resistive effect that the electric resistance varies in dependency upon a mechanical deformation are arranged on a single crystal substrate, and when an external force is exerted on a predetermined working point in an XYZ three-dimensional coordinate system and a mechanical deformation is produced by the external force, the sensor detecting a component in the X-axis direction Ax, a component in the Y-axis direction Ay, and a component in the Z-axis direction Az of an external force exerted on a working point on the basis of voltage values Vx, Vy and Vz obtained on the basis of a bridge circuit constituted by the plurality of resistance elements,
wherein coefficients K11, K12, K13, K21, K22, K23, K31, K32 and K33 are determined so that the relational equations expressed below hold between Ax, Ay, Az, Vx, Vy and Vz:
Ax=K11Vx+K12Vy+K13Vz
Ay=K21Vx+K22Vy+K23Vz
Az=K31Vx+K32Vy+K33Vz
and the values of terms of the right sides of the relational equations are computed by using analog multipliers, and a computation between respective terms of the right sides of the relational equations is performed by using analog adders/subtracters, thus to provide detected values Ax, Ay and Az from these computed results.
(4) The fourth feature resides in a signal processing circuit for an acceleration detector in which a plurality of resistance elements exhibiting the piezo resistive effect that the electric resistance varies in dependency upon a mechanical deformation, and when an external force is exerted on a predetermined working point, a mechanical deformation is produced on a single crystal substrate by the external force, the detector detecting a component in an X-axis direction Ax and a component in a Y-axis direction Ay perpendicular thereto of an external force exerted on a working point on the basis of respective bridge voltage values Vx and Vy of two sets of bridge circuits constituted by the plurality of resistance elements,
wherein coefficients K11, K12, K21 and K22 are determined so that the relational equations expressed below hold between Ax and Ay and Vx and Vy:
Ax=K11Vx+K12Vy
Ay=K21Vx+K22Vy
and the values of terms of the right side of the relational equations are computed by using analog multipliers, and a computation between respective terms of the right sides of the relational equations is performed by using analog adders/subtracters, thus to provide detected values Ax and Ay from these computed results.
In accordance with the above described signal processing circuit, a characteristic matrix showing the condition of interference produced between components in respective axial directions and an inverse characteristic matrix of the inverse matrix thereof are determined in advance. Further, corrective operations using this inverse characteristic matrix are carried out, thereby making it possible to cancel the influence of interference. Furthermore, since these corrective operations are all carried out at the analog computation circuit, the circuit configuration becomes simple and a correction circuit can be realized at a low cost. In addition, because operation is performed in an analog form, the operation speed becomes high, giving rise to no inconvenience even in the case of measuring an instantaneous phenomenon.
II Feature relating to the second object
To achieve the second object to provide a simple testing method with respect to each sensor, this invention has the following features.
(1) The first feature resides in a method of testing a sensor, the sensor comprising:
a strain generative body including a working portion adapted to undergo action of a force, a fixed portion fixed to a sensor body, and a flexible portion having flexibility formed between the working portion and the fixed portion,
a working body for transmitting an applied force to the working portion, and
detector means for transforming a mechanical deformation produced in the strain generative body by the transmitted force to an electric signal to thereby detect, as an electric signal, a force exerted on the working body,
wherein the method comprises the steps of determining a first portion and a second portion located at positions opposite to each other and producing a displacement therebetween, exerting a coulomb force between both the portions, and testing the sensor on the basis of the exerted coulomb force and a detected result by the detector means.
In accordance with the first feature, a coulomb force is exerted between the first and second portions. By this coulomb force, the first portion undergoes displacement relative to the second portion to induce a mechanical deformation in the strain generative body. Accordingly, the same state as the state where an external force is exerted on the working body can be created. Thus, test of the sensor can be carried out without actually applying an external force thereto.
(2) The second feature resides in, in the method having the above described first feature,
a method in which a first electrode layer is formed at the first portion and a second electrode layer is formed at the second portion to carry out a test conducted while exerting a repulsive force between the first and second electrode layers by applying voltages of the same polarity to the both electrode layers, respectively, and a test conducted while exerting an attractive force between the first and second electrode layers by applying voltages of polarities different from each other thereto, respectively.
In accordance with the second feature, by applying a voltage across two opposite electrode layers, a coulomb force can be exerted. In addition, by selecting the polarity of an applied voltage, the coulomb force can be exerted as either a repulsive force or an attractive force. Thus, the test having higher degree of freedom can be conducted.
(3) The third feature resides in a method of testing a sensor, the sensor comprising:
a strain generative body including a working portion adapted to undergo action of a force, a fixed portion fixed to a sensor body, and a flexible portion having flexibility formed between the working portion and the fixed portion,
a working body for transmitting an applied force to the working portion, and
detector means for transforming a mechanical deformation produced in the strain generative body by the transmitted force to an electric signal to thereby detect, as an electric signal, a force exerted on the working body,
wherein the method comprises the steps of determining a first plane surface and a second plane surface located at positions opposite to each other to produce a displacement therebetween by the action of the force, forming an electrode layer on the first plane surface, and forming, on the second plane surface, a plurality of electrically independent electrode layers at a plurality of portions,
the method further comprising the steps of applying a voltage of a first polarity to the electrode layer on the first plane surface, and selectively applying, every electrode layers, a voltage of a first polarity or a voltage of a second polarity opposite to the first polarity to the respective electrode layers on the second plane surface to exert a coulomb force of a repulse force or an attractive force between the electrode layer on the first plane surface and the electrode layers on the second plane surface, and thereafter conducting test of the sensor on the basis of the applied coulomb force and a detected result by the detector means.
In accordance with the third feature, since one electrode layer is divided into a plurality of subelectrode layers, an approach is employed to select the polarity of an applied voltage, thereby making it possible to carry out a test in which a coulomb force is exerted in various directions.
(4) The fourth feature resides in an acceleration sensor comprising:
a strain generative body including a working portion adapted to undergo action of a force, a first portion fixed to a sensor body, and a flexible portion having flexibility formed between the working portion and the fixed portion,
a weight body adapted to undergo action of a force by an acceleration applied to the sensor body, the weight body transmitting the force thus exerted to the working portion and allowing the stain generative body to produce a mechanical deformation,
a resistance element having the property that the resistance value varies in dependency upon a mechanical deformation produced in the strain generative body,
a first electrode layer formed on a first plane surface to produce a displacement by action of an acceleration,
a second electrode layer formed on a second plane surface opposite to the first plane surface, and
wiring means for connecting the resistance element, the first electrode layer and the second electrode layer to an external electric circuit,
to apply a predetermined voltage to the first and second electrode layers to exert a coulomb force between both the electrode layers, thereby permitting the strain generative body to produce a mechanical deformation even in the state where no acceleration is exerted.
In accordance with the fourth feature, within the acceleration sensor, two electrode layers for carrying out the test according to the above described first feature are formed, and wiring therefor is implemented. Accordingly, by only connecting a predetermined electric circuit to this acceleration sensor, the test can be carried out.
(5) The fifth feature resides in a magnetic sensor comprising:
a strain generative body including a working portion adapted to undergo action of a force, a fixed portion fixed to a sensor body, and a flexible portion having flexibility formed between the working portion and the fixed portion,
a magnetic body adapted to undergo action of a force by a magnetic field where the sensor body is placed, the magnetic body transmitting the force thus exerted to the working portion and allowing the strain generative body to produce a mechanical deformation,
a resistance element having the property that the resistance value varies in dependency upon a mechanical deformation produced in the strain gtenerative body,
a first electrode layer formed on a first plane surface to produce a displacement by action of a magnetic force,
a second electrode layer formed on a second plane surface opposite to the first plane surface, and
wiring means for connecting the resistance element, the first electrode layer and the second electrode layer to an external electric circuit,
to apply a predetermined voltage to the first and second electrode layers to exert a coulomb force between both the electrode layers, thereby permitting the strain generative body to produce a mechanical deformation even in the state where no magnetic force is exerted.
In accordance with the fifth feature, within the magnetic sensor, two electrode layers for carrying out the test according to the above described first feature are formed, and wiring therefor is implemented. Accordingly, by only connecting a predetermined electric circuit to this magnetic sensor, the test can be carried out.
(6) The sixth feature is directed to a sensor having the above described fourth or fifth feature, characterized in that one of the first and second electrode layer is constituted with a single electrode layer, and the other electrode layer is constituted with a plurality of electrically independent subelectrode layers, to select the polarities of voltages applied to the respective subelectrode layers, thereby permitting a mechanical deformation produced in the strain generative body to have directivity.
In accordance with the sixth feature, since one electrode layer is constituted with a single electrode layer, and the other electrode layer is constituted with a plurality of subelectrode layers, selection of the polarities of applied voltages is made, thereby making it possible to conduct a test in which a coulomb force is exerted in various directions.
(7) The seventh feature is directed to a sensor having the above described sixth feature, characterized in that the other electrode layer is constituted with two electrically independent subelectrode layers to select the polarities of voltages applied to respective subelectrode layers, thereby allowing the strain generative body to produce a mechanical deformation with respect to a direction of a line connecting the centers of two subelectrodes layers, and a mechanical deformation with respect to a direction perpendicular to the surfaces of the two subelectrode layers.
In accordance with the seventh feature, since two subelectrode layers are provided, a test in which a coulomb force is exerted with respect to two directions perpendicular to each other can be conducted.
(8) The eighth feature is directed to a sensor having the above described sixth feature, characterized in that the other electrode layer is constituted with four electrically independent subelectrode layers, and these subelectrode layers are arranged at respective end point positions of two orthogonal line segments so as to select the polarities of voltages applied to the respective subelectrode layers, thereby allowing the strain generative body to produce a mechanical deformation with respect to a direction of a first line segment of the two line segments, and a mechanical deformation with respect to a direction perpendicular to the surfaces of the four subelectrode layers.
In accordance with the eighth feature, since four subelectrode layers are provided in a crossing manner, a test in which a coulomb force is exerted with respect to three directions perpendicular to each other can be conducted.
(9) The ninth feature is directed to a sensor having the above described fourth or fifth feature characterized in that the first electrode layer and the second electrode layer are constituted with a plurality of electrically independent first subelectrode layers and a plurality of electrically independent second subelectrode layers, respectively, so as to select polarities of voltages applied to the respective subelectrode layers, thereby permitting a mechanical deformation produced in the strain generative body to have directivity.
In accordance with the ninth feature, selection having higher degree of freedom can be made. Thus, a test in which a coulomb force is exerted in various directions can be conducted.
III Feature relating to the third object
To achieve the third object to provide a sensor suitable for a higher sensitivity physical quantity measurement and a method of manufacturing such a sensor, this invention has the following features.
(1) The first feature resides in a sensor comprising:
a substrate wherein a working portion, a flexible portion and a fixed portion are defined substantially at the center of the substrate, around the working portion and around the flexible portion, respectively, to dig a groove in the flexible portion on the lower surface of the substrate, or to form a through hole in the flexible portion of the substrate to thereby allow the flexible portion to have flexibility and a resistance element is formed of which electric resistance varies on the basis of a mechanical deformation of the flexible portion on the upper surface of the substrate so as to detect changes in the electric resistance of the resistance element, produced on the basis of a displacement relative to the fixed portion of the working portion,
a working body for transmitting a force to the working portion being connected to the lower surface of the working portion,
a pedestal for supporting the fixed portion being connected to a first portion on the lower surface of the fixed portion,
wherein a second portion on the lower surface of the fixed portion and a portion on the upper surface of the working body being constituted so that they are opposite to each other with a predetermined spacing therebetween, a displacement in an upper direction of the working body being permitted to limitatively fall within a predetermined range by the second portion.
In accordance with the first feature, the central portion on the upper surface of the working body is connected to the lower surface of the working portion of the substrate, but the side portion of the working body extends to the portion below the fixed portion of the substrate. Accordingly, it is possible to make a design so that the volume of the working body is large as a whole. As a result, the weight of the working body is increased, thus making it possible to improve sensitivity with ease. Further, since the side portion of the working body extends up to the portion below the fixed portion of the substrate, it is possible to limit a displacement in an upper direction of the working body by making use of the fixed portion lower surface of the substrate as a control member. Accordingly, the necessity of individually providing a control member in an upper direction is eliminated, thus permitting the structure to be simple.
(2) The second feature is directed to a sensor having the above described first feature,
wherein the inside surface of the pedestal and the outside surface of the working body are constituted so that they are opposite with a predetermined spacing therebetween, thus making it possible to allow a displacement in a lateral direction of the working body to limitatively fall within a predetermined range by the inside surface of the pedestal.
In accordance with the second feature, in addition to the first feature, the inside surface of the pedestal and the outside surface of the working body are constituted so that they are opposite to each other with a predetermined spacing therebetween. For this reason, it is possible to limit a displacement in a lateral direction of the working body by making use of the inside surface of the pedestal as a control member. Accordingly, the necessity of individually providing a limit member in a lateral direction is eliminated, thus permitting the structure to be simple.
(3) The third feature is directed to a sensor having the first or second feature,
wherein the pedestal is fixed onto a predetermined control surface so that the control surface and the lower surface of the working body are opposite to each other with a predetermined spacing therebetween, thus making it possible to allow a displacement in a lower direction of the working body to limitatively fall within a predetermined range by the control surface.
(4) The fourth feature resides in a sensor comprising:
a substrate wherein a working portion, a flexible portion and a fixed portion are defined substantially at the center of the substrate, around the working portion and around the flexible portion, respectively, to dig a groove in the flexible portion on the lower surface of the substrate, or to form a through hole in the flexible portion of the substrate to thereby allow the flexible portion to have flexibility and a transducer is formed for transforming a mechanical deformation to an electric signal at the flexible portion on the upper surface of the substrate so as to detect changes in an electric surface produced on the basis of a displacement relative to the fixed portion of the working portion to thereby detect a physical quantity exerted on the working portion,
a working body for transmitting a force to the working portion being connected to the lower surface of the working portion,
wherein, when it is assumed that a perpendicular is drawn downward from the center of gravity G of the working body onto the upper surface of the substrate, the relationship expressed as L less than r holds between a length L of the perpendicular and a distance r from the foot P of the perpendicular up to the outside portion of the groove.
This fourth feature is based on the fact that the inventor of this application has found an optimum range in respect of a distance between a working point defined at the central portion on the upper surface of the substrate and the center of gravity of the working body. This optimum range satisfies such condition that sensitivities with respect to all directions substantially become uniform at the time of detecting a physical quantity in a three dimensional direction. For this reason, a sensor in which three is no difference between detection sensitivities dependent upon direction can be realized.
(5) The fifth feature resides in a method of manufacturing a sensor using resistance elements,
the method comprising the steps of:
defining a flexible area in the form of a square ring having a width on a first substrate,
defining a working area and a fixed area at one of the portion inside the square ring and the portion outside the square ring and at the other portion, respectively,
forming resistance elements within the flexible area on a first plane surface of the first substrate,
digging a groove in the form of parallel crosses in correspondence with the square ring position on a second plane surface of the first substrate to form, in the flexible area, a groove in the form of square comprised of a portion of the groove in the form of parallel crosses, thus allowing the flexible area to have flexibility,
connecting a first plane surface of a second substrate to the second plane surface of the first substrate, and
cutting the second substrate to thereby form a working body connected to the working area of the first substrate and comprised of a portion of the second substrate, and a pedestal connected to the fixed area of the first substrate and comprised of a portion of the second substrate.
In accordance with the fifth feature, a groove in the form of square is formed in the flexible area on the second plane surface of the first substrate. Since the groove in the form of square can be easily formed by digging a groove in the form of parallel crosses by mechanical processing, it is possible to efficiently form a precise groove. Further, the weight body or the magnetic body is formed by a portion of the second substrate, and the pedestal for supporting the first substrate is formed by another portion. Namely, prior to carrying out the dicing process, the weight body or the magnetic body and the pedestal can be formed every wafer.
(6) The sixth feature resides in a method of manufacturing a sensor using resistance elements,
the method comprising the steps of:
defining a plurality of unit areas on a first substrate, and, within each unit area, defining a flexible area in the form of a square ring having a width, and defining a working area and a fixed area at one of the portion inside the square ring and the portion outside the square ring and at the other portion, respectively,
forming resistance elements in each flexible area on a first plane surface of the first substrate,
digging a plurality of grooves on the side of a second plane surface of the first substrate in a longitudinal direction and in a lateral direction, respectively, to form four grooves along four boundary sides of the working area or the fixed area, thus allowing the flexible areas to have flexibility by these grooves,
connecting a first plane surface of the second substrate to the second surface of the first substrate,
cutting the second substrate to thereby form, within each unit area, a working body connected to the working area of the first substrate and comprised of a portion of the second substrate, and a pedestal connected to the fixed area of the first substrate and comprised of a portion of the second substrate, and
cutting off, every unit area, the first and second substrates to form sensors independent each other.
In accordance with the sixth feature, a plurality of unit areas are defined on the first substrate, and the processing proceeds at the same time with respect to respectively plurality of unit areas. Finally, one unit area will constitute one sensor unit. A plurality of grooves are dug in a longitudinal direction and in a lateral direction on the second plane surface of the first substrate, respectively. Within each unit area, respective four grooves are formed along four boundary sides of the working area or the fixed area. By these grooves, flexibility is given to the flexible area. Since it is sufficient to form grooves longitudinally and laterally in a matrix form, these grooves can be easily dug by mechanical processing. Thus, precise grooves can be efficiently formed.